This invention relates generally to a method and apparatus for welding steam turbine rotor shafts and, more particularly, to apparatus and method for axially aligning rotor shaft segments to be welded to form the shaft and maintaining alignment of the segments while welding.
Shafts of steam turbine rotors typically have very large radial dimensions, on the order of two to three feet, and may have an axial extent exceeding 30 feet. Because of such axial length and resulting weight, it may not be feasible to manufacture a shaft from a single integral piece of metal. Typically, two or more shaft segments, having a total axial dimension at least as great as the desired overall length of the shaft may be aligned and welded together to form the shaft. Additional machining may be required. During a welding procedure, shaft segments are rotated and weldment is typically applied by a stationary welding head which deposits weldment in the volume between opposing axial end faces of shaft segments to secure opposing axial end faces of segments together. Precise axial alignment between shaft segments to be welded together should be achieved prior to and maintained while welding to ensure integrity of the weld affixing adjacent opposing end faces of shaft segments together. If shaft segments are rotated relative to each other, any axial misalignment of adjacent end faces may cause the weld to crack or may produce inclusions within the weld. Additionally, rotation of the rotor while supported at axial positions outboard the weld may cause the weld to crack because of unequal stresses placed on the weld by the weight component force of the rotor. As is recognized by persons of ordinary skill in the art, inclusions in such welds are generally not acceptable due to stresses placed on the shaft during turbine operation. As used herein, the term "axial centerline" is meant to designate the center of rotation of the particular device or apparatus. A person of ordinary skill in the art recognizes that turbine rotors, comprising segments which have been welded together to form a rotor, have axial centerlines that may be within certain acceptable operational tolerances. In other words, the actual axial centerline must be within a predetermined distance from the "ideal" axial centerline. The term "axial centerline" referred to herein is meant to include such tolerances.
Before welding shaft segments together, the shaft segments must be preheated to a predetermined appropriate temperature for the weldment used. In a welding operation, the preheat temperature was about 500.degree. F. At that temperature, measuring means designed to maintain axial alignment by spanning the axial distance in the annular groove between adjacent end faces of the shaft segments, may be greatly affected by temperature so as to produce inaccurate or inconsistent results. It is important to maintain a circumferentially uniform axial distance between the opposing axial end faces of rotor segments to be welded in order to ensure axial centerline alignment between the two shaft segments.